Method of thermo-chemical treatment of cutting tools and plastic working tools

Abstract

The invention is addressed to preventing full reduction of oxides and enabling nitriding temperature through the oxidized layer to be raised which leads in a relatively shorter time, to formation of thicker and harder superficial layers, diffusion saturated with nitrogen. This aim has been achieved by carrying on the tool oxidation process advantageously for the whole time of the gas nitriding process, the said oxidation process accompanying the gas nitriding process being carried on in steam consisting 5 to 0.5 percent by volume of the whole gas atmosphere, the atmosphere of dissociated ammonia being obtained by the dissociation of ammonia at the presence of steam, the process of a simultaneous nitriding and oxidation of tools being carried on for an arbitrary period of time.

We claim: 
     
       1. A method of thermo-chemically treated cutting tools or plastic working tools made of high-speed, high-chromium and ledeburitic steels comprising the steps of forming an oxide layer on the surfaces of said tools, said layer having a thickness of being not greater than 6 μm, and said layer being produced by oxidizing said tools in steam at a temperature within the range of 520 to 570 degrees Centigrade for 30 to 120 minutes, and   gas nitriding the oxidized tools, for a period of time of at least 15 minutes in duration, in an atmosphere of dissociated ammonia at a temperature of 520 to 580 degrees Centigrade and simultaneously subjecting the oxidized tools to oxidation, in steam consisting of 5 to 95 percent by volume of the whole gas atmosphere, during any arbitrary interval of time occurring during said period whereby the atmosphere of dissociated ammonia is obtained by dissociation of ammonia in the presence of steam.   
     
     
       2. The method of claim 1 wherein the subjecting step is started, immediately after the oxide layer has been formed, without opening the furnace. 
     
     
       3. The method of claim 2 wherein the subjecting step is started after an arbitrary period of time has elasped after the oxide layer has been formed. 
     
     
       4. The method of claim 1, 2 or 3 wherein the forming, gas nitriding and subjecting steps are effected in a bed of bulk material.